In order to enable a Long Term Evolution (LTE) system to deploy transmission over an unlicensed spectrum, a Licensed Assisted Access (LAA) has been defined so far, that is, a terminal to access an unlicensed carrier shall firstly access a licensed primary carrier, where the unlicensed carrier can only operate as a secondary carrier.
Unlicensed spectrums have not been planned for any particular application system, but may be shared by various wireless communication systems, e.g., Bluetooth, WiFi, etc., where the various systems access shared unlicensed spectrum resources by preempting the resources. In order to guarantee transmission performance of an LTE terminal, and to make better use of an unlicensed band, the Listen Before Talk (LBT) mechanism shall be applied before data is transmitted in the unlicensed band. In an LBT process, the terminal performs Clear Channel Assessment (CCA) before it transmits data over a channel, to determine whether there is any other signal over the channel through energy detection so as to determine whether the channel is occupied or not, and only if the channel is idle, the terminal transmits data.
Since there are abundant unlicensed spectrum resources, the terminal may receive data concurrently over a large number of unlicensed carriers, and if an uplink control channel can only be transmitted over the licensed primary carrier, then there is a considerable overhead of uplink control channels over the licensed carrier, so an uplink control channel shall be transmitted over an unlicensed carrier. However, the resources over the unlicensed carriers are shared by the various systems, and the terminal shall firstly apply the LBT mechanism before it transmits data in the uplink, so the terminal may be unable to access a channel at certain time; and if the terminal fails to preempt a channel when it needs to transmit an uplink control channel, then uplink control information cannot be transmitted correctly in a timely manner.
At present, if an LBT check made over a corresponding unlicensed carrier to make Acknowledgement (ACK)/Negative Acknowledgment (NACK) feedback fails, then no feedback information can be transmitted.